Of the 143,000 ballot boxes used during Sunday's vote, 78,012 will be opened and the votes recounted, said Edmundo Jacobo, executive secretary of Mexico's Federal Electoral Institute.

Mr Jacobo said the recount could be finished by Thursday.

Mexico's electoral law states that the votes should be recounted if there are inconsistencies in the final tally reports, when the result shows a difference of one percentage point or less between the first and second place finishers or if all the votes in a ballot box are in favour of the same candidate.

With 99% of the vote tallied in the preliminary count, Enrique Pena Nieto of the Institutional Revolutionary Party, or PRI, led with 38% of the vote. Andres Manuel Lopez Obrador of the Democratic Revolution Party had 32%.

Authorities also will recount 61% of the ballot boxes in the vote for the Senate and 60% in the vote for the lower house of Congress, Mr Jacobo said.

Mr Lopez Obrador has refused to accept the preliminary vote tallies, saying the election campaign was marred by overspending, vote-buying and favourable treatment of Mr Pena Nieto by Mexico's semi-monopolised television industry.

The leftist candidate said Tuesday that his team had detected irregularities at 113,855 polling places, and called for a total recount.

Feeding suspicion of large-scale vote-buying were scenes of thousands of people rushing to grocery stores this week to redeem pre-paid gift cards they said the PRI had given them ahead of the vote. Several told reporters they had been told to turn in a photocopy of their voter ID card in order to get the gift cards.

Under Mexican election law, giving voters gifts is not a crime unless the gift is conditioned on a certain vote or is meant to influence a vote. However, the cost of such gifts must be reported, and cannot exceed campaign spending limits.